A typical fuel cell is a power generation system for producing electrical energy through a chemical reaction of oxygen and hydrogen contained in a hydrocarbon-based material such as methanol, ethanol, or natural gas. High specific energy of organic fuel, for example 6232 Wh/kg for methanol, renders the fuel cells attractive.
Fuel cells may be divided into the categories of alkaline cells, phosphoric acid cells, fused carbonate cells, solid oxide cells, or polymer electrolyte cells depending upon the type of electrolyte used in the cell. Polymer electrolyte membrane fuel cells (PEMFC) are a variety of polymer electrolyte cells. Compared to conventional fuel cells, the PEMFCs have superior power characteristics, lower operating temperatures, and faster start and response characteristics. As a result, the PEMFCs have a wide range of applications and can be used as transportable power sources for automobiles, distributed power sources for residences and public buildings, or small power sources for electronic devices.
A PEMFC essentially includes a stack, a reformer, a fuel tank, and a fuel pump. The stack forms the body of the PEMFC and the fuel pump provides the fuel stored in the fuel tank to the reformer. The reformer reforms the fuel to generate hydrogen gas and supplies the hydrogen gas to the stack, where hydrogen reacts with oxygen in an electrochemical reaction to generate electrical energy.
Alternatively, a fuel cell may be a direct methanol fuel cell (DMFC) in which liquid methanol fuel is directly introduced into the stack. Unlike a PEMFC, a DMFC does not require a reformer.
In the fuel cell system described above, the stack for generating the electricity may have several unit cells stacked adjacent to one another. Each unit cell includes a membrane-electrode assembly (MEA) and a separator, also referred to as a “bipolar plate.” The MEA includes an anode that is also referred to as a “fuel electrode” or an “oxidation electrode,” and a cathode that is also referred to as an “air electrode” or a “reduction electrode.” The anode and the cathode of an MEA are separated by polymer electrolyte membranes. The separators function both as channels, for supplying the fuel and the oxygen required for a reaction to the anode and the cathode, and as conductors, for serially coupling the cathode and the anode in the MEA, or for coupling the cathode of one MEA to the anode of a neighboring MEA. The electrochemical oxidation of the fuel occurs on the anode, and the electrochemical reduction of the oxygen occurs on the cathode. As a result of the transfer of the electrons generated by the oxidation/reduction reactions, electrical energy, heat, and water are produced.
A reformer used in a PEMFC system reforms and transforms water and a fuel containing hydrogen into the hydrogen gas needed to produce electricity in a stack. The reformer also removes substances such as carbon monoxide which poison a fuel cell and shorten its life. A conventional reformer includes a reforming reactor for reforming the fuel, and a carbon monoxide remover or a carbon monoxide reducer. The reforming reactor transforms the fuel into a reformed gas with abundant hydrogen through a catalytic reaction such as steam reformation, partial oxidation, or autothermal reformation. The carbon monoxide remover removes carbon monoxide from the reformed gas through catalytic reactions such as water-gas shift (WGS) or preferential oxidation (PROX), or by hydrogen purification using a separation membrane.
As described above, the conventional reformer generates hydrogen gas from the hydrogen-containing fuel through a chemical catalytic reaction using heat energy. Typically, the reformer includes a heat source for generating the heat energy, a reforming reactor for generating hydrogen gas from the fuel using the heat energy, and a carbon monoxide remover for reducing the level of carbon monoxide included with the hydrogen gas.
In the reformer of a conventional fuel cell system, the reforming reactor has a catalytic reformation layer for reforming a mixed fuel obtained by mixing liquid-phase fuel with water. Through a catalytic reforming reaction in the catalytic reformation layer, the reforming reactor can generate a reformed gas with abundant hydrogen from the mixed fuel that is heated to a certain predetermined temperature.